Creating a better commute for wildlife: Understanding the importance of establishing and maintaining wildlife habitat corridors

Introduction

Imagine there's two islands, and on each of these two islands is an all-inclusive resort.  Imagine that the only way to get to these islands is to swim.   Island A is only a short swim away across shallow water while Island B is a 50 mile doggy-paddle off of the coast (although the water is at least still warm and calm in this scenario).  Which all-inclusive resort do you think is going to get more patronage: the one on Island A or the one on Island B?

Now image instead of people trying to swim across an ocean to a beach resort, image wildlife trying to reach their homes - but only this time the ocean they must cross consists of busy freeways, asphalt parking lots and polluted land.  Which island are most of them going to make it to...and how long can they survive on a habitat island surrounded by all of this chaos?  Chances are, not for long, and those islands will soon be void of most wildlife species.

This unusual sounding situation is actually an all too common scenario across natural landscapes.  The phenomena that I'm referring to is known as habitat fragmentation.  

What is island biogeography?

Habitat fragmentation, although occasionally occurring naturally, is often the product of man-made development such as urban development, intensive agriculture, or the construction of travel right-of-ways such as freeways(Akcakya et al, 2007).   It's consequences are often complex.  One thing to understand is that wildlife needs three components to maintain a sustainable population:  (1) food, (2) shelter, (3) water, and (4) space. The combination of these needs which help to maintain wildlife populations is known as wildlife habitat.  

In an ideal situation, an area of wildlife habitat is large and diverse in its architecture.  For example, in Figure 1A, there is a nature preserve that is surrounded by intact woodland areas and a small prairie opening.  Species diversity, or number of species per unit area*, is high meaning that there are lots of different wildlife (and plant) species in an area that has lots of good habitat.  Over time, however, human involvement in the landscape increases, and an area of land is cleared for agriculture and a farm pond is constructed (Figure 1B).   Believe it or not, the diversity of species (both plants and animals) actually increases, at least for a short time, due to the fact that the landscape itself is increased in its structural diversity.  The landscape has now four different habitat types - forest, prairie, open cornfield, and a pond.  Overtime, development continues, and in Figure 1C the addition of a large cornfield eliminates the prairie area and further agricultural development combined with the construction of a freeway leaves only the nature preserve and some of the surrounding land intact.  Diversity of plant and wildlife species is declining as there is less space and quality of habitat.  In Figure 1D, further development eliminates all naturalized land outside of the nature preserve, and the nature preserve itself is then subjected to invasive species, pollution, and as we shall see, viable populations of some wildlife. Overall diversity of species is low.

Figure 1: A common scenario across the North American landscape. (A) A nature preserve surrounded by an intact natural ecosystem. Species diversity is normal. (B) Agricultural fields, a road, and the creation of a pond create a temporary increase in species diversity as the result of a more diverse landscape. (C) Species diversity starts declining as habitat is lost and remaining habitat becomes too small but for all of the most hardy plants and animals. (D) Development finally surrounds the entire nature preserve, and the nature preserve itself is no longer ecologically valuable.  Species diversity is minimal.

If you noticed, the example illustrated above shows that even though the land inside the nature preserve is never developed, it nonetheless declines in its species diversity - and as I mentioned, its quality of wildlife habitat.  Why does this happen to the nature preserve when everything that's going on is occurring outside of the nature preserve boundaries?

During the 1960s, two young ecologists, Robert MacArthur and E.O. Wilson, studied wildlife populations on islands in the Caribbean (and maybe they went to some of those all-inclusive resorts that I was talking about?).  Over the course of their research, the began to notice an interesting pattern:  the number of different species seemed to increase with the size of the island.  In other words, larger islands had more different kinds of wildlife species (Miller, 2007) (see Figure 2).

Figure 2:  A frequent observation among wildlife biologists is that
 large habitat islands contain a higher number of a given species.

As MacArthur and Wilson continued their ecological research on wildlife populations, they began to formulate a theory which became known as island biogeography.  

Island biogeography is based on the assumption that there are source and sink populations of a given species - and  in this context we will discuss this concept in terms of wildlife populations, even though much if this theory is relevant to plants and other living organisms.  Wildlife populations in an area that is fragmented often exhibit a metapopulation dynamic (Akcakya et al, 2007).  Wildlife metapopulations are best explained as such:  There is a close relationship between the amount of habitat (area of land) and the stability of a wildlife population.  In a large tract of naturalized land, not unlike what is illustrated in Figure 1A, there is a relatively even amount of wildlife emigration and immigration.  In other words, this large tract of land has an even rate of wildlife arriving to the area and leaving the area (or going extinct).  Wildlife biologists refer to this area of land as a source, which provides a source population of wildlife.

Take a look at Figure 3, which shows a representation of a source population - or in the context of island biogeography, a mainland (such as a continent or one of the larger islands in the Caribbean that MacArthur and Wilson researched).

Figure 3:  In an island biogeography context, wildlife populations are dispersed from a source population to sink populations.  A sink can be thought of as a small island off of the mainland, whose supply of wildlife populations is dependent on immigration from a large mainland.  See text for a more detailed explanation.  

The source population helps "supply" the smaller islands with species such as different kinds of birds.  These smaller islands are the sinks for the wildlife population at hand.  The island that is nearby to the mainland (sink A) has a high rate of immigration of wildlife from the source, in part because birds and other wildlife don't have to travel too far to get to their next area of suitable habitat (or going back to the earlier metaphor, they don't have to swim too far to get to the island beach resort).  Sink A has a higher rate of extinction because there is a lot of competition for space.  Sink B on the other hand, is an island that is pretty darn far from the source population, and not too many animals have the chance to migrate that far.  Sink B also has a much higher rate of extinction because the island is small AND there just aren't that many individuals.  This is common among wildlife metapopulations in that their stability is density dependent, or is dependent on how many individuals there are and in what amount of habitat there is available to them. This density dependent factor is one of the reasons the nature preserve in Figure 1 declined in its species diversity.  There just wasn't enough space or immigration from other nature preserves (and pollution and the encroachment of invasive species was also a factor in the decline of habitat quality).

Why does it matter?
So how does the metapopulation model for wildlife pertain to wildlife populations in a fragmented landscape? The illustration I drew in Figure 3 shows islands in a sea, but those islands could just as easily be - and often are, habitat islands in an urban landscape.

Figure 4 shows a satellite image of part of Washington, DC.  Near the northwest corner of this image, you'll notice a large greenspace.  This is Rock Creek Park.  You should also notice several other large greenspaces, especially near the upper-right hand corner of the image.  Those are the many other large parks and natural areas around Washington, DC.

Figure 4:  Google satellite image of a section of Washington, DC.  

As DC grew over time, these protected greenspaces remained, essentially becoming "islands" of natural habitat in an urban area (urban areas in the image are shown in white and grey).  These isolated areas of open land can be thought of as islands similar to what MacArthur and Wilson examined - only this time wildlife must immigrate across a "sea" of concrete, asphalt, busy highways, landfills, and other areas far more inhospitable than  the nice calm ocean described in our earlier example.  In fact, it's possible that not too many of these parks can be considered true sources of wildlife population - in fact all of them could indeed be sinks in a state of gradual ecological decay (see Figure 1).

What can be done?
Despite the complexity of ecological fragmentation, there are management methods that can minimize the adverse effects of urban development.  Take a look at Figure 5, which shows protected parcels of land in a Washington, DC suburb.  You'll notice several large tracts of conservation land and several smaller parcels of preserves that are separated from each other by urban and suburban development.

Figure 5:  Protected parcels of land (outlined in red) in a suburb of Washington, DC.   Modified from original image produced by the Maryland Department of Natural Resources,  2013.   

Most of the parcels are isolated, and are indeed "habitat islands."  Many of them have suffered ecological decay similar to the hypothetical scenario I presented earlier in Figure 1.  How can we connect these islands so that there can be a steadier immigration and emigration routes for wildlife?  In other words, how can we make it easier for species to "swim" across an "ocean" of urban development so that they can populate those islands more easily?

To ease the adverse effects of habitat fragmentation and increase the chance that most wildlife species will have a safe modes of travel, many communities across the country are establishing wildlife corridors, or linear areas of naturalized land that connect to larger naturalized areas (Figure 6).  In areas where natural green-space is at a premium, the establishment of native landscaping and more environmentally friendly property management practices can aid in the creation of a wildlife corridor (Figure 7).

Figure 6: Cleveland's "Emerald Necklace" is a system of interconnected parks that connect greenspaces and wildlife habitat across an urban and suburban landscape.  

Figure 7: Naturalized landscaping can aid in the establishment of wildlife corridors in highly urban areas that lack additional greenspaces, such as this environmentally-friendly office building in Indianapolis.  

Conclusion
The challenge to reconnect our natural resources is ongoing and often an uphill battle.  The idea of developing and maintaining wildlife corridors is not an exact science, and even our best efforts to keep intact wildlife metapopulations could fail.  The idea, however, of protecting nature by "setting aside" isolated tracts of nature preserves and letting them be is a concept that is not only antiquated, but as determined by recent ecological research, detrimental to protecting our natural diversity of plant and animal species.

"Simply put: small isolated woodlots cannot sustain many woodland bird, reptile, and amphibian species. To preserve biodiversity for all of our native forest species, we need to supplement the relatively common woodlands that still exist all across the east (young, disturbed, rapidly growing forests that are timbered) with a quilting of restored mature forests in large, undisrupted preserves."  (Arc of Appalachia, 2014)

We need to focus our efforts on the big picture and assume an ecological approach to land management. Otherwise, our habitat islands could become smaller and smaller, adversely affecting not just wildlife populations - but people too!


References
Akcakya, H. R., Mills, G., & Doncaster, P. (2007). The role of metapopulations in conservation. In D. Macdonald & K. Service (Eds.), Key topics in conservation biology (pp. 64-84).

Arc of Appalachia. (2014). Why preserve America's eastern forest?. Retrieved from http://arcofappalachia.org/arc/why.html

Maryland Department of Natural Resources. Maryland Department of Natural Resources, (2013). Forest stewardship map for the city of Greenbelt; county: Prince George's

Miller, G. T. (2007). Essentials of ecology. (4th ed., p. 143). Toronto, Ontario: Thompson Brooks/Cole.

Notes
* There are many ways of measuring species diversity, but this is a discussion that is beyond the scope of this blog post.